Method of maintaining the efficiency of phosphate coating baths used at room temperature



c 2,863,793 v Patented Dec. 9, 1958 No Drawing. Application April 17,

Serial No. 578,592

Qlaims priority, application April 20, 1955 4 Claims. (Cl. 148-6.15)

This invention relates to a method of maintaining the efficiency of phosphate coating baths used at room temperature. Phosphate coating processes such as the Bonderizing and Parkerizing processes have become of considerable importance inthe modern technique of surface protection of metals, particularly of ferrous metals. Those processes are usually carried out at increased temperature of about 40 to 45 C., using phosphate coating baths of various compositions commonly based on zinc phosphate and zinc nitrate at given ratios, which may also contain other salts as accelerators, such as copper and nickel salts. Cold phosphate coating processes are also known (see German Patent No. 900,163). The latter processes utilize baths containing accelerators and having a pH at least 0.5 higher than those operating at higher temperatures. In such processes, the baths are similar to hot phosphate coating baths, except that, as stated, the pH is raised to neutralize part of the acidity so that the hydrolysis equilibrium shifts to room temperature. However, when using cold phosphate coating baths, in order to accelerate the formation of the coating, the coating baths must contain an excess of the oxidizing agents, so that a P O /NO ratio of 1:2 is normally used in these baths.

in case of cold phosphate coating baths, the feed salts (that is, the salts that are added to the baths to replenish the amounts of salts which have been removed) must also contain a much higher amount of oxidizing agent than that required for hot baths. Nevertheless, in practical use these cold baths have always been found to be of unsatisfactory permanency in strength. In fact, it has been found that, for example, 1 liter of such a bath, after phosphate coating therewith an area of 3 sq. meters and after regenerating the same three times by adding feed liquor having a P O /NO ratio of 1:1.4, is no longer capable of producing a good coating because the P O /NO ratio had risen to 1:7 during the coating operations.

Now I have found that a cold phosphate coating bath can be made to remain efficient during a very great number of operations, even for more than one year of use, when employing feed salts at a different ratio than that of the bath; in other words, feed salt solutions containing lower amounts of oxidizing agent (N0 than those used previously and having, therefore, a composition approaching that of the feed salts for hot baths.

However, I found that if the baths are to maintain the necessary metal-attacking power at normal temperature, another suitable oxidizing agent, such as sodium nitrite, must be added, from time to time or continuously, in amounts not exceeding of the N0 comprised in the feed salts. The addition of the foregoing amount, which maintains the concentration of the oxidizing nitrites, is an indispensable condition but one should not go above this established limit, if the efficiency of the bath is to be'preserved. Accelerators in the form of copper or nicltel salts may be added to the bath.

The advantages of the present invention in comparison with prior processes will beshown in the following examples which are presented to illustrate the present invention, but not to limit the scope of the appended claims.

Example 1 Panel sheets of soft-iron are dipped and phosphate coatedin 1 liter of a phosphate coating bath containing gr. Zn, 20 gr. P 0 and 40 gr. N03 (thus having a P O /NO ratio of 1:2). After five consecutive series of such sheets, representing a total area of 1 sq. meter, have been phosphate coated, the bath is regenerated (by adding so much feed liquor to the bath until 10 cc. of the latter consume the same number of cc. of N sodium hydroxide with phenolphthalein as the end point indicator as had been consumed by 10 cc. of the bath before beginning the phosphate coating operation). The feed liquor contains gr./ 1. Zn, 170 gr./-1. P 0 240 gr./l. N0 0.1 gr./l. Cu; thus having a P O /NO ratio of 121.41.

.NaNO is also added with the feeding solution, at a proportion of 50 gr. per each liter.

After using 1 liter of the bath for phosphate coating 3 sq. meters of soft-iron sheets which, therefore, includes three intermittent regenerations by means of the foregoing feed liquor, the bath must be considered unsuitable for further service since it yields quite poor phosphate coatings.

Example 2v A bath is employed as specified in Example 1, having the same initial composition and using the same operating temperature on a similar series of soft-iron sheets. Likewise, regeneration (feeding) is effected each time the bath (1 liter) has been used for phosphate coating 1 sq. meter of sheets. However, according to the present in vention, the feed liquor contains 170 gr./l. Zn, 240 gr./l. P 0 170 gr./l. N0 and 0.1 gr./l. Cu. Thus, it has a EO /N0 ratio of 1:0.73. Discontinuous or continuous addition of sodium nitrite is effected in amounts never in excess of hi of the quantity of N0 added with the feeding solution.

After 1 liter of the bath has been used for phosphate coating 4 sq. meters of sheets and, therefore, has been regenerated for four times with the foregoing solution, it continues indefinitely to give a protective coating of excellent properties, similar to the coatings applied to the panels constituting the first square meter.

In these two examples, the variations of the concentration of P 0 clue to the coating action and the feeding, are as follows:

In the first example, the P O /NO ratio has gone down, after phosphate coating 3 sq. meters of sheet material and after three regenerations, from 1:2 to 1:7, whereas in the second example, after an initial reduction to 122.5, the P O /NO ratio remains substantially constant.

Example 3 A bath is used which is similar to that of Example 2. The same procedures are followed, except that sodium nitrite is neither present in the initial bath nor introduced with the feeding solution. Even the first phosphating operation results in a coatingwhich is entirely inadequate a rust preventative.

in the foregoingexamples, the baths and the feeding solutions are made up from zinc phosphate, zine nitrate and/er an alkali metal nitrate, while the oxidizing agent added to the feeding solution consists of sodium nitrite.

While in the examples the P O /NO ratio of the initial bath has been given as 1:2 and, in Example 2, the HO /N0 ratio of the feeding solution as 1:0.73, it is to be understood that, according to this invention, the EO /N0 ratio in theinitial solution may range from 1:1 to 1:2 and in the feed solution from 1:1 to 120.5, in order to maintain throughout the coating operations a P O /NO ratio ranging from 1:1 to 1:2.5 in the bath.

1 claim:

1. The method of maintaining the efiiciency of phosphat: coating baths used to coat a metallic surface in the cold, comprising preparing, as coating bath, an aqueous solution of zinc phosphate and at least one nitrate selected from the group consisting of alkali metal nitrates and zinc nitrate, and during the metal coating process maintaining in said coating bath a P O /NO ratio ranging from about 112.5 to about 1:1 by replenishing the coating oath depleted in the coating operations with a feed liquor comprising an aqueous solution of zinc phosphate, at least one nitrate selected from the group consisting of alkali metal nitrates and zinc nitrate, the P O /NO ratio of the feed liquor being within a range from about 1:1 to about 1:0.5, and also adding sodium nitrite as oxidizing agent in an amount not more than about one tenth of the weight of N0 contained in the feed liquor.

2. The method of claim I, wherein the P O /NO ratio of the feed liquor is adjusted between 120.7 and 120.8.

3. The method of claim 2, wherein said coating bath and said Iced liquor comprise as accelerators small amounts of a salt selected from the group consisting of copper and nickel salts.

4. The process of claim 1 in which the feed liquor comprises zine phosphate and zinc nitrate, the amount of s dium nitrite being about one tenth of the weight of N0 contained in the feed liquor, the i O /NQ, ratio of the initial coating bath being in the ratio range from 1:1 to 1:2.

References Cited in the file of this patent UNITED STATES PATEIJTS 

1. THE METHOD OF MAINTAINING THE EFFICIENCY OF PHOSPHATE COATING BATHS USED TO COAT A METALLIC SURFACE IN THE COLD, COMPRISING PREPARING, AS COATING BATH, AN AQUEOUS SOLUTION OF ZINC PHOSPHATE AND AT LEAST ONE NITRATE SELECTED FROM THE GROUP CONSISTING OF ALKALI METAL NITRATES AND ZINC NITRATE, AND DURING THE METAL COATING PROCESS MAINTAINING IN SAID COATING BATH A P235/NO3 RATIO RANGING FROM ABOUT 1:2.5 TO ABOUT 1:1 BY REPLENISHING THE COATING BATH DEPLETED IN THE COATING OPERATIONS WITH A FEED LIQUOR COMPRISING AN AQUEOUS SOLUTION OF ZINC PHOSPHATE, AT LEAST ONE NITRATE SELECTED FROM THE GROUP CONSISTING OF ALKALI METAL NITRATES AND ZINC NITRATE THE P2O5/NO3 RATIO OF THE FEED LIQUOR BEING WITHIN A RANGE FROM ABOUT 1:1 TO ABOUT 1:0.5, AND ALSO ADDING SODIUM NITRITE AS OXIDIZING AGENT IN AN AMOUNT NOT MORE THAN ABOUT ONE TENTH OF THE WEIGHT OF NO3 CONTAINED IN THE FEED LIQUOR. 